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1989 911 Carrera Resonant Engine Noise

Engine

Vehicle Information: Model: 911 carrera; Year: 1989; Body Type: Coupe; Engine (type, size, modifications): none; Total Mileage: 123,700;

This question concerns my '89 Carrera. The car currently has over 123,000 miles and is a daily driver. I typically drive 4-5 miles each way to and from work. Last week I drove 150 freeway miles one afternoon and at my destination, the city speeds were somewhat lower ( 35 MPH). I noticed that between about 2200 and 2800 RPM, that there was a harsh resonant sound. This was my first observation as it had never happened with commuter driving. The noise/sound occurred with or without the clutch engaged and in frist, second, and third gears (I did not get to 4th) before reaching my destination and shutting the car off. It sat overnight and I did not experience the same sound as the day before when I initially began driving. After driving (freeway again) about 60 miles, I stopped for gas and observed the same harsh resonant sound at the same RPM range. This happened again when I arrived at my home following the remianing 90 miles of the trip. Both coming and going, there were no gauge alerts. Engine temp., oil pressure,etc were all normal. There was no apparent loss of performance during the 2200-2800 RPM period that generated the sound. Now, I know that there is supposed to be a whistle around 2400 if all the relays are working properly and I am not able to hear that as distinctly as I have in the past. So, my question is: is this electronic and the relay is overheating or is it mechanical or is it exhaust? The only thing that would make me think that it may be exhaust is that I did smell something acrid momentarily during the freeway driving but attibuted that to envornmental noxiousness. This was transient. I have not repeated the highway drive to see whether this is repeatable.

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